Conventionally, semiconductor lasers are used for small coherent light sources in various technologies, e.g., optical communication, optical measurement, and optical fabrication. However, semiconductor lasers in general have problems, e.g., small optical output, wavelengths and light amounts vulnerable to change in temperature, and increased spectrum in width in accordance with greater driving current supplied by a semiconductor laser for greater optical output. In contrast, external cavity semiconductor lasers achieve stable optical output that provide the feedback of a part of the laser light (a certain wavelength of light) emitted by a semiconductor laser device and amplify a specific wavelength of light. For example, Patent Document 1 proposes a configuration where only a specific wavelength of backbeam emitted by a semiconductor laser device is returned to the semiconductor laser device by a lens and a holographic diffraction grating.
An external cavity semiconductor laser making use of the high output characteristics of a gain waveguide semiconductor laser has been proposed recently. Laser light are emitted from a common surface in two different directions that are offset from a center axis of a waveguide in a laser device in a gain waveguide semiconductor laser (grain-waveguide diode laser). Thus, an external cavity semiconductor laser using a gain waveguide semiconductor laser provides feedback of one laser light to a semiconductor laser device and makes use of the other laser light, i.e., output light. Non-Patent Documents 1 and 2, and Patent Document 2 describe examples of external cavity semiconductor lasers using a gain waveguide semiconductor laser.
FIG. 17 illustrates an external cavity semiconductor laser disclosed by the Non-Patent Document 1. Laser lights 108 and 109 are emitted in two different directions from a first end surface 2 of a semiconductor laser device 101 of a gain waveguide semiconductor laser so that the laser lights 108 and 109 are offset from an optical axis 108 of a waveguide in the semiconductor laser device 101. Provided to collimate the two laser lights are a fast axis collimator (FAC) 102 and a slow axis collimator (SAC) 103. One of the laser light, i.e., the laser lights 108 and 109 having passed through the fast axis collimator (FAC) 102, slow axis collimator (SAC) 103, and an aperture 105a are reflected by a high-reflection (HR) mirror 104 and further passed through the aperture 105a, the slow axis collimator (SAC) 103, and the fast axis collimator (FAC) 102, then returned to the semiconductor laser device 101 to form an external cavity laser light. The other one of the light, i.e., the laser light 109 passing through the fast axis collimator 102, the slow axis collimator 103, and an aperture 105b and being emitted is an output laser light.
In the case of external cavity semiconductor laser illustrated in FIG. 17, the semiconductor laser device 101 has a reflecting coating (reflecting unit) 106 on a second end surface 101d disposed opposite the first end surface 101c. The laser lights 108 and 109 reflected by the reflecting mirror 104 and returned to the semiconductor laser device 101 are further reflected by the reflecting coating 106, and then emitted again from the first end surface 101c. A one-half part 101a of an active layer of the semiconductor laser device 101 is an optical path of an external cavity that emits one of the laser light, i.e., the laser light 108 forming the external cavity laser light. Another half part 101b is an optical path that emits the other output laser light 109.
The external cavity semiconductor laser disclosed by Non-Patent Document 2 obtains laser light having a decreased width of spectrum. As illustrated in FIG. 18, laser lights 118 and 119 are emitted from an external cavity semiconductor laser in two directions that are offset from the optical axis of a waveguide in a semiconductor laser device 110. Configured for the laser light 118 is an external cavity optical system where a part of the laser light emitted from a semiconductor laser device 110 passes through a fast axis collimator (FAC) 111, a lens 112, and an aperture 113, and corrected to parallel light by a lens 114. The corrected parallel light is adapted to a specific wavelength of light by an etalon 115 and a diffraction grating 116. In FIG. 18, a design idea is shown for increasing wavelength selectivity by adjusting the etalon 115 and adjusting the angle of the diffraction grating 116. Meanwhile, a reference numeral 117 in FIG. 18 indicates a reflecting unit that introduces the output laser light 119 to the exterior thereof.
The basic configuration of the external cavity semiconductor laser disclosed by Patent Document 2 is the same as that shown in FIG. 18. As illustrated in FIG. 19, a portion 124 of the laser emitted by a semiconductor laser 120 is incident into a cylindrical lens 121 followed by an etalon 122 and a diffraction grating 123, and a reflected light 125 is returned to the semiconductor laser 120. Another laser light emitted by the semiconductor laser 120 becomes output laser light 126 to be radiated.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. S58-71687    [Patent Document] WO03/055018A1    [Non-Patent Document 1] Volker Raab, et al., “External resonator design for high-power laser diodes that yield 400 mW of TEM00 power”, p167-169, vol.27, No.3, OPTICS LETTERS, Feb. 1, 2002    [Non-Patent Document 2] Volker Raab, et al., “Tuning high-power laser diodes with as much as 0.38 W of power and M2=1.2 over a range of 32 nm with 3-GHz bandwidth,” p1995-1997, Vol.27, No. 22, OPTICS LETTERS, Nov. 15, 2002